The present invention relates to electronic devices in which epitaxial layers of different materials are grown on substrates and in particular relates to Group III nitride layers that are formed, at least initially, on dissimilar substrates such as silicon carbide and sapphire as precursors to devices including, but not limited to, light emitting diodes (LEDs).
The term light emitting diode is used herein in a relatively broad sense to describe those semiconductor devices that produce photons when a current is applied across the device, and typically, although not exclusively, across a p-n junction that is formed by adjacent layers of p-type and n-type materials. The general structure and operation of light emitting diodes is well understood in this art and is described in numerous well-understood and widely available references.
Light emitting diodes have found application in a wide variety of devices and circumstances. Among other applications, they are used for indicator lights, in simple alphanumeric display devices (e.g., calculators), in full color video display screens, as backlighting for other types of displays (e.g., in conjunction with liquid crystal displays) and more recently as illumination sources. As solid-state devices, they share the advantages of light weight, relatively inexpensive cost, small size, generally high reliability and long lifetime.
The Group III nitride compounds have become increasingly of interest in the production of light-emitting diodes and related photonic devices. Group III refers to the third group of the periodic table of the elements and includes (among other elements) gallium (Ga), aluminum (Al), and indium (In). When these elements are formed in binary, ternary, or quaternary compositions, they exhibit desirable semiconductor properties. In particular, the Group III nitrides have relatively large band gaps, which gives them the capacity to emit relatively high energy photons, which in turn means that they can produce photons in the green, blue, violet, and ultraviolet portions of the electromagnetic spectrum. This favorably distinguishes them from materials such as gallium phosphide that have smaller band gaps and that thus emit longer wavelength, lower energy photons in the red and yellow portions of the spectrum.
Blue (and higher-frequency) emitting materials are of particular interest because they can be combined with red-emitting and green-emitting materials to produce white light. Stated more fundamentally, producing white light from LEDs (and in the absence of other materials such as phosphors) requires a combination of blue, red, and green light.
Additionally, the relatively high energy of blue, violet or ultraviolet photons can help stimulate certain phosphors, which in turn produce light in a different, more desired frequency. For example, photons in the blue portion of the visible spectrum can typically excite certain phosphors which emit yellow frequencies. When the frequency of the emitting semiconductor and the phosphor are properly selected, the combination of blue and yellow light can produce white light.
Additionally, controlling the composition (atomic fractions) of certain Group III nitrides (e.g. InGaN) determines the particular wavelength of the emitted photons. Thus, in a certain sense the Group III nitride compositions can be tuned to produce emissions of desired colors.
As another favorable characteristic, the Group III nitrides are “direct” emitters meaning that all of the energy produced from an electronic transition is emitted as a photon. This is in contrast to other wide band gap materials such as silicon carbide in which the transitions are indirect; i.e., the transition produces some energy in the form of a photon and some energy as vibrations. Accordingly, other factors being equal, direct emitters such as the Group III nitrides produce light more efficiently than do indirect emitters such as silicon carbide.
The wide bandgap advantages of Group III nitrides are not, however, limited to light emission or to devices such as LEDs. Wide bandgap semiconductors have advantages in many types of semiconductor electronic devices including power transistors and high frequency devices such as MESFETs and HEMTs.
To date, however, the growth of bulk crystals of Group III nitrides remains an academic exercise rather than a practical reality. Thus, practical devices that incorporate Group III nitrides for light emitting (or other) purposes typically incorporate epitaxial layers of Group III nitrides on substrates formed of other materials. In the field of light-emitting diodes (as well as some other devices) these substrate materials tend to be either silicon carbide (SiC) or sapphire (Al2O3).
Sapphire offers the advantage of being mechanically strong and highly transparent. Sapphire cannot, however, be conductively doped. Thus, devices formed on sapphire substrates cannot be arranged in a “vertical” orientation. As known to those familiar with this art, a vertical orientation is one in which the electrical (ohmic) contacts are at axially opposite ends of the device. Vertical devices offer certain engineering and size advantages in comparison to “horizontal” devices in which the ohmic contacts must be arranged in side by side geometry because of the lack of a conductive substrate. Generally speaking, for the same effective device area, the footprint of a vertical device is smaller than the footprint of a horizontal device.
Silicon carbide offers the advantage of being conductive, but is more difficult to make transparent, particularly when doped to obtain the desired conductivity. Nevertheless, because some devices purposely incorporate insulating substrates, both types of substrates (sapphire and silicon carbide) are of theoretical and commercial interest for LEDs and other devices.
As a structural factor, however, the unit cell dimensions of the Group III nitrides differ somewhat from that of silicon carbide or sapphire; e.g., hexagonal GaN is 3.19 Å, hexagonal SiC is 3.08 Å and sapphire is 2.75 Å. Thus, growing Group III nitride layers on silicon carbide or sapphire substrates always includes a lattice mismatch. In turn, this lattice mismatch produces a resulting strain in the Group III nitride layer. Such strain normally falls into two categories, tension and compression. In layman's terms, a layer under tension has been stretched somewhat to match the substrate. A layer under compression has been squeezed to match the substrate. When a thin layer of Group III nitride is placed on one of these substrates, the resulting strain is smaller and less relevant. As the Group III nitride layer becomes thicker, however, the strain normally increases and causes additional problems such as dislocations and cracking.
Furthermore, although the issues of tension and compression generally arise in the context of adjacent layers that are different from one another, impurities—including dopants—can cause strain even in otherwise homogeneous materials and even in the absence of an adjacent different material.
Accordingly, Group III nitride device structures on SiC or sapphire substrates typically include one or more Group III nitride buffer (i.e., lattice transition) layers between the substrate and the active layers. These transition layers typically have a composition different from the active layers, but with a lattice matched more closely to that of the relevant substrate or deposited in such a way that strain (thermal or mechanical) associated with the subsequent layer deposition is compensated by the properties of the buffer layer. In order to produce vertical devices on conductive substrates (e.g., SiC), any such buffer layers are typically produced to be conductive and thus include dopants. For number of reasons, however, such dopants can increase the resulting strain in a crystal layer. In many circumstances, the Group III nitrides are normally doped with silicon. In turn, silicon has a tendency to alter the lattice constant of the doped Group III nitride in a manner that increases strain and unfavorably promotes cracking in the epitaxial layer stack. As any of these transition layers become thicker, the defects and stress problems generated by the layer-substrate mismatch will tend to increase. As noted above, thinner transition layers can be pseudomorphic or more nearly pseudomorphic; i.e., they are sufficiently elastic to match the substrate with minimal resulting strain and minimal defect formation. As the layers grow thicker, however, the crystal structure of the epilayer reduces the elasticity and increases the resulting strain. If the strain becomes too great, defects are formed in the epitaxial layer stack in order to reduce it. These can be either micro-scale defects such as point or line defects or more gross defects such as epitaxial layer cracking or delamination. Either class of defects can be detrimental to device performance and overall yield of the LED manufacturing process.
Thus, from the strain standpoint, thinner transition layers would normally be preferred. In some current device technology, however, the dissimilar substrate (e.g. SiC or sapphire) is eventually removed in order to produce the final structure. In those cases, the transition layer serves at least three purposes: (i) it provides the crystal transition noted above while the substrate is present, (ii) it provides a structural support for the active layers after the substrate has been removed, and (iii) it provides a path for lateral current spreading from the n-contact to the device. When serving as a structural support, a thicker transition layer is sometimes desired or necessary rather than a thinner one. Thicker layers help provide the required support and manufacturing tolerance during the production of the finished device. As another factor, thicker epitaxial layers can help in current spreading, especially when doped in a way that provides for low resistivity.
Accordingly, a need exists for techniques and structures that favorably permit or include thicker transition layers with high doping levels while minimizing the disadvantageous strain that can accompany such thicker layers.